U.S. patent number 5,693,745 [Application Number 08/598,405] was granted by the patent office on 1997-12-02 for method for synthesizing polyamic acid for manufacturing flexible amorphous silicon solar cell.
This patent grant is currently assigned to Industrial Technology Research Institute. Invention is credited to Wen-Yueh Hsu, Jinn-Shing King, Lee-Ching Kuo, Yu-Tai Tsai.
United States Patent |
5,693,745 |
Kuo , et al. |
December 2, 1997 |
**Please see images for:
( Certificate of Correction ) ** |
Method for synthesizing polyamic acid for manufacturing flexible
amorphous silicon solar cell
Abstract
The present method provides a method for preparing the PI
varnish which has the steps of: 1) preparing a mixed solution of
60-100% by weight aprotic solvent, and 0-40% by weight aromatic
solvent; 2) adding into the mixed solution in a mole ratio of 1:9
two aromatic diamines; and 3) further adding in the mixed solution
in a mole ratio of 1:5 two aromatic dianhydrides. Such PI has a
suitable thermal expansion coefficient and characteristics
different form those of the PI currently in use.
Inventors: |
Kuo; Lee-Ching (Hsinchu,
TW), King; Jinn-Shing (Taipei, TW), Hsu;
Wen-Yueh (Hsinchu, TW), Tsai; Yu-Tai (Taichung,
TW) |
Assignee: |
Industrial Technology Research
Institute (Hsinchu, TW)
|
Family
ID: |
21898487 |
Appl.
No.: |
08/598,405 |
Filed: |
February 8, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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214856 |
Mar 18, 1994 |
|
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38176 |
Mar 26, 1993 |
5356656 |
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Current U.S.
Class: |
528/350; 528/125;
528/126; 528/128; 528/174; 528/183; 528/229; 528/353; 427/69;
427/504; 427/503; 427/497; 427/496; 528/220; 528/179; 528/171;
257/E31.042; 427/58; 528/173; 528/185; 427/487 |
Current CPC
Class: |
B05D
7/24 (20130101); H01L 31/202 (20130101); H01L
31/03921 (20130101); Y02E 10/50 (20130101); Y02P
70/50 (20151101); Y02P 70/521 (20151101) |
Current International
Class: |
H01L
31/20 (20060101); H01L 31/0392 (20060101); H01L
31/18 (20060101); H01L 31/036 (20060101); C08G
073/10 (); B05D 005/00 (); C08F 002/46 (); C08J
003/00 () |
Field of
Search: |
;528/353,125,126,128,173,171,174,179,183,185,220,229,350
;427/58,69,487,496,497,503,504 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Seidleck; James J.
Assistant Examiner: Hampton-Hightower; P.
Attorney, Agent or Firm: Christie, Parker & Hale,
LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
The present application is a continuation of application Ser. No.
08/214,856, filed Mar. 18, 1993, abandoned, which is a divisional
application of the parent application bearing Ser. No. 08/038,176
and filed on Mar. 26, 1993, which issued as U.S. Pat. No. 5,356,656
on Oct. 18, 1994.
Claims
What we claim is:
1. A method for preparing a polyimide film for manufacturing
flexible amorphous silicon solar cells comprising the steps of:
preparing a mixed solvent of an aprotic solvent and an aromatic
solvent;
adding into the mixed solvent two aromatic diamines to obtain a
solution;
adding into said solution two aromatic dianhydrides;
reacting the resulting solution in a nitrogen atmosphere under
50.degree. C. to obtain a random copolymeric polyamic acid of a
random copolymeric polyamic acid;
applying a varnish coating of said polyamic acid on a glass
substrate;
imidizing said polyamic acid varnish coating into said polyimide
film on said glass substrate by multiple heat treatment steps;
depositing in vacuum a metal film on said polyimide film and having
said metal film patternized;
depositing in vacuum an amorphous silicon film on said metal film,
and having said amorphous silicon film patternized;
depositing in vacuum a transparent electrode film on said amorphous
silicon film, and having said transparent electrode film
patternized;
applying a transparent protective film over said transparent
electrode film;
separating said polyimide film from said glass substrate; and
applying a protective film on a back side of said polyimide
film.
2. A method according to claim 1 wherein the mixed solvent
comprises at least about 60% by weight aromatic solvent and less
than about 40% by weight aromatic solvent.
3. A method according to claim 1 wherein said aromatic diamines
have a molar ratio of 1:9.
4. A method according to claim 1 wherein said aromatic dianhydrides
have a molar ratio of 1:5.
5. A method according to claim 1 wherein the step for reacting the
resulting solution is carried out at room temperature.
6. A method according to claim 1 wherein the step for reacting the
resulting solution is carried out for six hours.
7. A method according to claim 6, wherein the aprotic solvent is
one selected from a group consisting of N-methyl-2-pyrrolidone,
N',N-dimethyl acetamide, and N,N'-dimethyl formamide.
8. A method according to claim 6, wherein the aromatic solvent is
selected from the group consisting of toluene and xylene.
9. A method according to claim 1 wherein said pyromellitic
dianhydride, said 3,3',4,4'-benzophenone tetracarboxylic acid
dianhydride, said 3,3',4,4'-oxy diphenyl tetracarboxylic acid
dianhydride and said 3,3'4,4'-diphenyl tetracarboxylic acid
dianhydride have a common chemical structure of ##STR5## and have
their Rs respectively as follows: ##STR6## .
10. A method according to claim 6 wherein said two aromatic
diamines are selected from the group consisting of phenyl diamine,
diamino diphenyl sulfide, diamino diphenyl ether, 4,4'-diamino
diphenyl methane, diamino diphenyl sulphone, diamino toluene, and
diamino diphenyl which have a common chemical structure of:
wherein R' is selected from the group consisting of: ##STR7## .
11. A method according to claim 1 wherein said aprotic solvent is
selected from the group consisting of N-methyl-2-pyrrolidone,
N,N'-dimethyl formamide, and mixtures thereof, and said aromatic
solvent is selected from the group consisting of toluene, xylene,
and mixtures thereof.
12. A method according to claim 1 wherein said two aromatic
dianhydrides are selected from the group consisting of pyromellitic
dianhydride and 3,3',4,4'-benzophenone tetracarboxylic acid
dianhydride, pyromellitic dianhydride and 3,3',4,4'-oxy diphenyl
tetracarboxylic acid dianhydride, 3,3',4,4'-benzophenone
tetracarboxylic acid dianhydride and 3,3',4,4'-oxy diphenyl
tetracarboxylic acid dianhydride, 3,3',4,4'-benzophenone
tetracarboxylic acid dianhydride and 3,3',4,4'-diphenyl
tetracarboxylic acid dianhydride, and 3,3',4,4'-oxy diphenyl
tetracarboxylic acid dianhydride and 3,3',4,4'-diphenyl
tetracarboxylic acid dianhydride.
13. A method according to claim 1 wherein the step for imidizing
the polyamic acid varnish coating is carried out by heating the
polyamic acid at about 100.degree. C. for 30 minutes, about
200.degree. C. for about 30 minutes, about 300.degree. C. for about
30 minutes, and about 350.degree. C. for about one hour.
Description
BACKGROUND OF THE INVENTION
Solar cell is a new pollution-free energy source with applications
and market demand thereof increasing steadily. It finds
applications in consumer electronics products as well as in power
systems, and has good prospects of being one of the major energy
sources. Flexible amorphous silicon solar cells are fabricated by
depositing amorphous silicon, metallic and transparent conductive
oxide films on a flexible substrate with the desirable features of
light weight, thin thickness, flexibility, portability and
nonfracture, and can be more useful than the traditional
glass-substrate amorphous silicon solar cells.
There are two main types of flexible amorphous silicon solar cells,
namely:
(1) The ones with metal substrate:
Presently, the substrate is made of stainless steel on which there
are sequentially grown metallic, amorphous silicon, and transparent
conductive oxide (TCO) films, as typified by the products of ECD,
Sovonics Solar System companies of the U.S.
(2) The ones having the substrate made of a polymer material:
Two kinds of polymer films are used, including films of polymer
materials with good transparency and those with poor transparency.
The former has a very high transmission in the visible light range
so that there can be sequentially grown thereon transparent
conductive oxide, amorphous silicon and metal films to form a
structure the same as that of the glass-substrate solar cell,
purchased from Sanyo Electric Co. Ltd. Amorphous silicon solar
cells with the substrate made of a polymer material with poor
transparency have the same structure as that of metal substrate
amorphous silicon solar cells. On the amorphous silicon solar
cells, there are 10 sequentially grown metal amorphous silicon, and
transparent conductive oxide films, as typified by the solar cells
developed by the Iowa Thin Film Technology Inc. of the U.S., and
the Sanyo Electric Co., Ltd. of Japan.
Compared with polymer substrate solar cells, metal substrate solar
cells are heavy and not easily rollable. This is because it is not
easy to make metal substrate thin and rollable enough. The flexible
solar cells are required to be light-weight, thin as well as
flexible. Although European Patent Application No. 0,189,976 filed
by Sovonics Solar System has disclosed a very thin, light weight
and flexible amorphous silicon solar cell on a very thin metal
substrate (<50 .mu.m), or by etching away the metal substrate
after the cell is produced, the manufacturing procedure thereof is
too complicated, and the cost is too high. Solar cells with polymer
substrate will be more flexible, lighter in weight, and thinner
than those with metal substrate.
The polymer films suitable for being used as the substrate of solar
cells should have the following characteristics: (1) good heat
stability; (2) good mechanical strength; (3) surface smoothness and
thickness uniformity; (4) high purity (low ion content) to avoid
gas release in a high vacuum, and (5) good weatherability to avoid
degrading after long time exposure to sunlight. Therefore,
developing a suitable polymer material for the substrate would be
one of the key techniques. Besides, the choice of the right
technology for depositing the films on a polymer substrate in a
vacuum is also critical because of the significant thermal
expansion coefficient of polymer films. The right technology would
ensure that the polymer substrate would not be deformed or warped,
and the films deposited thereon would be even in thickness and
would not be easily stripped off or cracked. The types of polymer
substrates for polymer film solar cells include:
1) polyimide (PI)-film substrate:
Most of the PI films used are those marketed under the trademark of
"Kapton" from Du Pont of the U.S., having primary constituents of
pyromellitic dianhydride (PMDA) and oxydianiline (ODA), and with
desirable characteristics in heat resistance, mechanical strength,
electrical insulation, radiation-shielding, and
chemical-resistance. However, its coefficient of thermal expansion
(TCE) is relatively high (4.0-5.0.times.10.sup.-5 cm/cm .degree.C.)
and the PI film is very soft, and when aluminum (TCE:
1.3.times.10.sup.-5 cm/cm .degree.C.), chromium (TCE:
6.times.10.sup.-6 cm/cm .degree.C.), and amorphous silicon (TCE:
1.9-4.times.10.sup.-6 cm/cm .degree.C.) films are deposited on the
PI film in a vaccuum (with an operation temperature up to
250.degree. C.), there would be a stress mismatch between the PI
film and the films deposited on it, resulting in the deposited
films being easily stripped off and cracked, and uneven in
thickness.
To alleviate the problems of warping and excessive thermal
expansion coefficent of PI films used as a substrate, some people
tried to add a supporting layer under the PI film as disclosed in
U.S. Pat. No. 4,541,583; Japan Kokai Nos. 60-79779 and 60-66869;
and European Patent Application No. 0,189,976 filed by Sovonics
Solar System. This supporting layer is mostly a thin film of a
metal. Some use fabrics or polymer material. The European Patent
Application No. 0,189,796, filed by Sovonics Solar System, uses a
very thin metal film on which a PI film is applied. When the films
are depposited on the substrate, the supporting layer is
selectively etched to get the flexible solar cell. Alternatively,
the metal film can be retained as a part of the substrate. The
method according to the Sovonics Solar System patent is too
complicated. The composite substrate comprising a thin metal
supporting layer and a PI film is still too soft, and its thermal
expansion coefficient would still be excessive, and the substrate
would be warped during the a-Si:h deposition.
A supporting layer made of fabrics or glass fiber fabric would not
make a good substrate since it has a surface having a roughness
possibly amounting to several .mu.m to several ten's .mu.m and the
PI film applied thereon will also be uneven.
2) Polyether sulfone (PES) film-substrate:
PES has a glass transition temperature of 225.degree. C. and will
easily release gas in a vacuum. Thus PES substrate (TCE) has poor
characteristics and is not suitable for a high efficiency device.
The TCE of PES is 5.5.times.10.sup.-5 cm/cm .degree.C. which is
even larger than that of PI and presents more difficulty in
reaching the stress match between PES film and metal, amorphous
silicon films and indium tin oxide (ITO, a transparent electrode
material).
3) PI/MeO/PI/MeO (wherein MeO means metal oxide) multi-layer film
substrate:
Laminated substrate comprising alternating PI and MeO layers would
ameliorate the softness and reduce the TCE of PI substrate.
However, the lamination process is too complicated. Until now,
solar cells with polymer substrate and metal substrate are mostly
fabricated by a roll-to-roll process to deposit the various layers
on the flexible substrate in vacuum. The equipment, such as the
thermal evaporator sputter or the E-Beam gun evaporator, required
for depositing the metal or the transparent conductive oxide films
on substrate, although having been commercialized, is very
expensive. And it is complicated and difficult to adapt to
roll-to-roll plasma-enhanced chemical vapor deposition (PECVD),
also called glow discharge chemical vapor deposition (GD-CVD)
required for depositing amorphous silicon films. Presently, no
commercialized PECVD equipment available is used for the
roll-to-roll operation. Users must develop the equipment required
by themselves. To enhance the efficiency of the solar cells, the N,
I, P layers of amorphous silicon would have to be deposited
separately in three different reaction chambers (i.e. in a
multichamber PECVD equipment) to avoid contaminating each other.
It's very difficult to make such solar cells by the roll-to-roll
process, which is still under studies now.
Therefore, the following drawbacks exist in depositing the films of
polymer solar cells by roll-to-roll process: 1) The available
equipment for the glass substrate solar cells cannot be utilized,
and the required new equipment is not available; 2) Because the
film deposition occurs in a large area, it is difficult to maintain
the required flatness of the polymer substrate in this large area
film deposition process. It is also difficult to control the
deformation of the substrate caused by heating, resulting in
expansional deformation of the films deposited and poor quality; 3)
It is difficult to perform multichamber with PECVD to increase the
efficiency of polymer solar cells; 4) The equipment is expensive,
and consequently the production cost is high.
The kapton from Du Pont, as mentioned earlier, has a high thermal
expansion coefficient and has strong adhesion between the glass
substrate and the PI film deposited thereon, rendering it difficult
to have the PI peeled off the substrate.
It is therefore attempted by the Applicant to deal with the above
shortcomings encountered by the prior art.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
simple and cost-effective method for manufacturing a flexible
amorphous silicon solar cell in which the PI film/solar cell can be
peeled from the glass plate, after having deposited or applied the
various thin layers on the PI substrate formed on the glass
plate.
Another object of the present invention is to synthesize a kind of
PI used as a substrate for a flexible amorphous silicon solar cell,
with a suitable thermal expansion coefficient, and characteristics
different from those of the PI currently in use. Also the PI
substrate with the solar cell films deposited thereon can be easily
peeled off the glass plate.
A further object of the present invention is to provide a method
for manufacturing a flexible amorphous silicon solar cell, whereby
the existent equipment for manufacturing the glass substrate solar
cells can be utilized without requiring any modification to be
performed on the PECVD and laser scribing parts to effect cost
saving.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematical view of the solar cell according to the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, a method for manufacturing a
flexible amorphous silicon solar cell includes the following steps:
a) applying a coating of a PI varnish on a glass substrate, the
varnish being polyamic acid, the precursor of polyimide; b)
imidizing the PI varnish to form a PI film (2) on the glass
substrate, c) depositing in vacuum a metal film (3) on the PI film
and patterning it; d) depositing in vacuum an amorphous silicon
film (4) on the metal film and laser scribing the silicon film; e)
depositing in vacuum a transparent conductive oxide film (5) on the
amorphous silicon film, and patterning it; f) applying a
transparent protective coating (6) over the transparent conductive
oxide film; g) separating the PI film from the glass substrate by
cutting the PI film around the periphery of the glass substrate and
peeling it off; and h) providing a protective film (7) on the back
surface of the PI film, as shown in FIG. 1. Incident light enters
the solar cell through the transparent protective coating (6).
The metal films include an Al layer of 2000 .ANG. and a Cr layer of
1000 .ANG.. The amorphous silicon films include an n layer of
300-600 .ANG., an i layer of 4,000-5,000 .ANG., and a p layer of
100-200 .ANG..
When producing solar cells of large areas, every layer of the vapor
deposited thin film needs to be patternized, and laser scribing is
the patternizing technique known to be the most convenient, widely
used and adaptable to quantity production. However, if the
substrate itself is soft, it would be difficult to have it flatly
spread over the scribing platform, resulting in imperfect flatness.
If the substrate is not flat, the scribing result would be poor.
Thus, it would be difficult to laser scribe large area solar cells
without a firm, flat supporting surface beneath it. Since the
present invention can directly form a PI film on a glass substrate,
it is therefore possible to get a flat PI/glass substrate upon
which the various layers of thin films are grown. So, the laser
scribing can be carried out just as it is on an ordinary glass
substrate. The equipment currently in use for glass substrate solar
cells can be utilized without modifications, and cost saving is
thus effected. The manufacturing method according to the present
invention is easy to use. By applying a PI coating on a glass
substrate, a flat PI film is spreaded thereon. The various layers
of thin films deposited on the PI film will not be warped, and the
stress difference between the films because of the TCE variance
will be abated, and a film with good flatness and adhesion is
obtained. Because the conventional PI film substrate has a
relatively large thermal expansion coefficient, and after being
deposited thereon various layers of thin films, it exhibits a
strong adhesiveness to the glass substrate and it is difficult to
be stripped therefrom. Such PI film is not suitable to be used for
the present invention. The PI used for this invention is different
from traditional ones, and is synthesized from a special
formulation, and has all of the required characteristics mentioned
above. For example, after the various layers of thin films are
deposited thereon, the PI film can be easily stripped off the glass
substrate. The traditional PI is synthesized by using a single
species of the aromatic dianhydride, and a single species of
aromatic diamine. Whereas, according to the present invention, we
use two species of aromatic dianhydrides and two species of
aromatic diamines to synthesize the PI. The polyamic acid serving
as the precursor of the polyimide for the present flexible
amorphous silicon solar cell can be synthesized-as follows:
The chemical compounds used for the synthesization include:
1) two aromatic dianhydrides selected from: pyromellitic
dianhydride, 3,3',4,4'-benzophenone tetracarboxylic acid
dianhydride, 3,3',4,4'-oxy-diphenyl tetracarboxylic acid
dianhydride, and 3,3',4,4'-diphenyl tetracarboxylic acid
dianhydride, each of which has a common chemical structure of:
##STR1## wherein R is selected from the following group:
##STR2##
2) two aromatic diamines selected from: phenyl diamine, diamino
diphenyl sulfide, diamino diphenyl ether, 4,4'-diamino diphenyl
methane, diamino diphenyl sulphone, diamino toluene, and diamino
diphenyl each of which has a common chemical structure of
wherein R' is selected from the following group: ##STR3##
The synthesizing procedures are as follows:
1) Preparing a solution by mixing 60-100 wt % aprotic solvent and
0-40 wt % aromatic solvent. The aprotic solvent which can be used
includes: (a) N-methyl-2-pyrrolidone, (b) N',N-dimethyl acetamide
or (c) N,N'-dimethyl formamide. Whereas, the aromatic solvent that
can be used include cyclohexanone, toluene, xylene, acetone,
methylethyl ketone, or r-butyrolactone;
2) Adding into the mixed solution any two monomers of the aromatic
diamines mentioned above with a molar ratio of 1:9;
3) Adding further into the mixed solution obtained any two species
of the above-mentioned aromatic dianhydride with a molar ratio of
1:5. The reaction is allowed to proceed under room temperature and
a nitrogen atmosphere for six hours to obtain polyamic acid, the
precursor of polyimide, also known as the PI vanish which can be
applied or coated on a glass substrate, by spin coating, extrusion
die casting or doctor blade method. The coating is then imidized to
result in a PI film with even thickness, and is flatly spreaded
over the substrate with characteristics as tabulated in Table
I.
TABLE I
1. Compounds for synthesization: BPDA/BTDA/PPDA/ODA
2. BPDA: BiphenyI tetracarboxylic acid dianhydride
3. BTDA: Benzophenone tetracarboxylic acid dianhydride
4. PPDA: P-Phenyl diamine
5. ODA: Oxy dianiline
6. Mixed solvent solution: N-methyl-2-pyrrolidone (NMP)
7. The solid content of NMP/xylene: 15%
8. The solid content of NMP/toluene: 20%
9. Break-down voltage: 4500 v/mil
10. TCE: 20.5.times.10.sup.-6 .degree.C..sup.-1
11. On-set decompose temperature: 450.degree. C.
12. Tensile strength: 13.7 kg/mm.sup.2
13. Elongation: 29%
14. Dielectric strength: 3.3 (10 KHZ)
15. Volume resistivity: >10.sup.16 .OMEGA.-cm
16. Surface resistivity: >10.sup.15 .OMEGA.
17. Water absorption rate: 1.2% (dipped in water for 24 hours)
Table II shows the peel strength between the kapton PI (from Du
Pont) film and the glass substrate. The kapton PI is synthesized
essentially from the two monomers PMDA ##STR4##
The peel strength between the kapton film and the glass substrate
is 0.01 lb/in, if no film is deposited on the PI film in vacuum.
The peel strength will remain unchanged if the PI filmed is just
vacuum heated for a time period under the depositing temperature
for depositing the metal film without actually depositing metal
film. If the metal film is deposited, the peel strength will
increase to 0.6. That is to say, if the adhesion between the PI
film and the glass substrate is increased, we cannot perfectly
separate the PI film from the substrate. The more layers of
deposited films there are, the stronger the adhesion will be. For
example, after depositing the metal film, amorphous silicon film
and transparent conductive oxide films, the peel strength will be
increased to 1.37 lb/in. And, it is then more difficult to
perfectly separate the PI film from the glass. In contrast, the
adhesion between the .PI film according to the present invention
and the glass is so small that it is out of the testing range of
the tensile machine and the peel strength cannot be measured.
Although the PI film is well adhered to the glass substrate during
the depositing procedure, when all layers of thin films are
deposited on the PI film, we can still cut the PI film around the
substrate to easily separate it from the the glass substrate, in
order to obtain an intact film without any cracks.
TABLE II ______________________________________ filmed state of
substrate Peel strength (lb/in)
______________________________________ 1. PI film/glass (without
heating 0.01 in vacuum) 2. PI film/glass (vacuum heated for a 0.01
time period and at a temperature for depositing the metal film) 3.
metal film/PI film/glass 0.6 4. transparent conductive oxide film/
1.37 amorphous silicon film/metal film/ PI film/glass
______________________________________
Example I
1.3716 g (0.0127 mole) 1,4-phenyl diamine and 22.86 g (0.1143 mole)
4,4-diamino diphenyl ether were fully dissolved into a mixed
solvent composed of 180 g N-methyl-2-Pyrrolidone and 120 g xylene.
6.5430 g (0.0203 mole) 3,3',4,4'-benzophenone tetracarboxylic acid
dianhydride was then added into the solution obtained. Then 22.1488
g (0.1016 mole) pyromellitic dianhydride was added into the
solution. After 6 hours of reaction carried out in the solution
under room temperature and the nitrogen atmosphere, a tawny
polyamic acid having 15% solid content was obtained. A doctor blade
with a 300 um clearance was used to apply the obtained polyamic
acid on a glass substrate. The coating was then imidized at
100.degree. C. for 30 minutes, 200.degree. C. for 30 minutes,
300.degree. C. for 30 minutes, and 350.degree. C. for one hour, to
obtain a PI layer having a smooth and fine surface, and a thickness
of 30 um.
Example II.
On the PI/glass substrate, there were sequentially deposited by an
E-Beam Gun Evaporator or a Sputter, an Al film of 2000 .ANG., a Cr
film of 1000 .ANG., and by PECVD three layers (i.e. n, i, p) of
amorphous silicon films having thicknesses: n: 300 .ANG.-600 .ANG.;
i: 4000 .ANG.-5000 .ANG.; and p: 100 .ANG.-200 .ANG.. The reaction
gases were the n layer: PH3 and SiH4; the i layer: SiH4; the p
layer: B.sub.2 H.sub.6, H.sub.2, CH.sub.4, and SiH.sub.4. The
reaction temperature was 250.degree. C., the reaction pressure was
0.3-0.5 torr, and the RF power density was 0.1 w/cm.sup.2. Finally,
one layer of indium tin oxide transparent electrode with a
thickness of 2000 .ANG. was deposited by an E-Beam Evaporator or
Sputter, and an amorphous silicon solar cell was obtained. Then, a
coating of silicon resin was applied on the amorphous silicon solar
cell to form a protective film. After cutting the PI film around
the substrate, the PI filmed solar cell can be peeled from the
substrate.
In summary, the PI used for the present flexible amorphous silicon
solar cell is synthesized from a special formulation with
characteristics different from the traditional ones, and with a
reasonably low TCE value. With several layers of thin films
deposited thereon, the PI film can still be easily separated from
the glass. The flexible amorphous silicon solar cell according to
this invention, can be cost-effectively produced by making direct
use of the existent equipment for producing glass substrate solar
cells without any modification to be performed on the amorphous
silicon depositing and laser scribing parts therein.
While the above provides a complete description of the invention,
it will be appreciated that alternate modifications or equivalents
may be applied without departing from the spirit and scope of the
invention. Therefore, the scope of the invention is not limited by
the above description, but is defined by the appended claims.
* * * * *